The invention relates to a filter insert including a material permeable to fluid and having a number of planar fold walls arranged in a substantially continuous zigzag shape and including depressions and/or elevations stamped into the plane of at least a part of a respective one of the fold walls in order to stiffen the fold walls, the depressions and/or elevations of adjacent fold walls being connected to one another at least partially and being supported on one another.
Filter inserts of the above types are used to remove contamination from a fluid flowing through a filter, particularly air or industrial gasses, but theoretically liquids as well. At present, filter elements consisting of a micro fibre fleece, particularly glass fibres, are predominantly used in air filters.
In order to increase the effective filter area relative to the inflow surface of the air filter which forms a filter element or filter area, the filter medium is folded in a zigzag shape so as to produce a number of folds adjoining one another via fold edges, which are located at an acute angle to the walls of the folds and through which the medium to be purified flows at right angles to the direction of the fold edges.
As a result of the depositing of material, and particularly the settling of larger particles, on the inflow side of the filter, turbulence in the inflowing fluid, slight irregularities in the folds, etc, inhomogeneities may occur in the fluid flow as the fluid flows through the filter, thereby subjecting the fold arrangement to strong mechanical alternating stresses and possibly bringing about deformation of the fold arrangement.
In order to hold the folds at a specified distance from one another and mechanically stabilise the fold arrangement of the filter insert, the folds are therefore provided with projections protruding from the plane of the fold walls, as described for example in U.S. Pat. No. 3,531,920 or, according to another special embodiment, in DE 41 26 126 A1, in such a way that the projections of adjacent fold walls abut on one another and the fold walls support one another. In order that the fold walls are directed at an inclined angle to the incoming fluid, ie. the fold layers are substantially triangular in cross section, the projections must also be approximately triangular or trapezoidal in cross section.
These arrangements have proved suitable for filter inserts with a fold height up to about 100 mm, with relatively tightly packed folds. Beyond this fold height and with larger fold spacings, however, the depth of impression required is so great that, with conventional filter materials, there is the risk of the folds being punched through, thereby unacceptably increasing the number of rejects.
It is also known from DE 40 38 966 to place separate spacers with the same function on the fold walls or to insert said spacers therein. The spacers may be adhesive aggregates, particularly in fibre form, as mentioned as a possibility in the specification referred to above or as illustrated in DE 30 37 019 A1. In addition to acting as spacers, they also have the effect of connecting the fold walls and thus further increasing the rigidity of the filter insert.
The adhesive is applied before the folding of the filter material onto the flat strip, and the fold walls are adhered by the contact of the adhesive threads with one another during the folding operation. This solution is therefore only suitable for very densely packed folds. In addition, the fold layers formed in this way tend to have a meandering configuration in cross section, which does not lead to optimum flow qualities.
DE 39 03 730 A1 describes how an adhesive thread which joins the fold layers together and stabilises them and which may be applied to the edge area after folding, is combined with impressions in the fold walls. This solution results in mechanically very stable filter inserts, but is subject to essentially the same restrictions as the solutions without adhesive in terms of the fold height and fold spacing which can be achieved.
DE 42 06 407 describes how the fold edge area of a previously folded length of filter material is covered with a fine web of adhesive threads in order to join the folds together and stabilise the filter insert. This solution is no longer practical for larger fold spacings since the fine adhesive web cannot produce a sufficiently stable attachment of fold edges which are spread further apart and the filaments of the web "sag" (particularly when applied from below to a folded material located above during the manufacturing process) and do not assume a defined position relative to the fold edges.
From EP 0 377 419 A1 an arrangement is known in which adhesive aggregates of varying sizes (larger at the top and smaller at the bottom) located in the top and bottom areas of a fold wall arrangement which is to be formed are to be used to adhere walls of folds in a manner such as to produce a substantially triangular cross section of fold. However, larger fold spacings and heights cannot be achieved by this method because extremely bulky adhesive aggregates would be required which would greatly reduce the effective filter surface.